Mechanical surface smoothing of micron-sized iron powder for improved silica coating performance as soft magnetic composites
Peter Slovenský, P. Kollář, Nanxuan Mei, Miloš Jakubčin, A. Zeleňáková, Maroš Halama, Inger Odnevall Wallinder, Yolanda S. Hedberg
Abstract
Producing defect-free and durable coatings that can withstand elevated annealing temperatures is important when preparing soft magnetic composites. A mechanical surface smoothing method was employed on water atomized Fe powder as an alternative to surfactants, before applying a SiO2 coating via the surfactant-free Stöber method. This study evaluated the effect of mechanical surface smoothing and the number of SiO2 coating layers on iron powders of different magnetic properties compared with non-coated iron powders. Electrochemical, microscopic, spectroscopic, and electro-magnetic methods were used to characterize the coating- and the magnetic properties. Sulphur and manganese were present in the outermost (5–10 nm) surface oxide of the non-smoothed angular iron powders, whereas absent in the case of the smoothed, more spherical particles. The surface coverage of the SiO2 coatings and the magnetic properties were significantly improved for the surface-smoothed iron powders compared to the non-smoothed reference powders. The lack of electrochemical signal from iron oxides of the SiO2-coated smoothed particles indicated close-to-complete surface coverage of the coating, also confirmed by electron microscopy. Signals from SiO2 and organic residues of the coating procedure increased with the number of coating procedures. The compacted surface smoothed Fe/SiO2 powder showed substantially reduced total energy losses compared to compacted non-smoothed reference powders.